In the production of steel, a ferrous melt is typically produced in a suitable furnace and then tapped into a ladle where it is treated with one or more ingredients for refining or alloying purposes. It is well known to add calcium to the molten ferrous material at this point as a refining agent for oxide inclusion flotation, oxide inclusion morphology modification, desulfurization, etc. Unfortunately, the low density (relative to steel), volatility and reactivity of calcium severely complicate the task of providing a satisfactory process for its addition to the molten material in the ladle.
A variety of techniques have been employed for the addition of calcium to the molten material in a steelmaking ladle. Bulk addition of calcium-containing particulate materials is unsatisfactory because these materials rapidly rise to the surface of the melt without spending a sufficient residence time therein. Efforts to increase residence time by pouring the particulate material directly into the tapping stream from the furnace give rise to excessive reaction of the calcium with atmospheric oxygen. Introductions of calcium-containing materials by plunging or the injection of clad projectiles into the melt generally provide adequate residence times but are complicated, expensive and time-consuming procedures. It has also been proposed to inject calcium-containing powders into a melt by inert gas injection through a refractory lance. Since sizable flows of gas are required to propel the powder into the molten ferrous material, a high level of turbulence is generated at the surface of the melt as the gas is released, thereby causing an excessive exposure of the molten ferrous material to oxygen and nitrogen in the atmosphere. Furthermore, after leaving the lance, the calcium tends to rise rapidly through the melt in the inert gas plume surrounding the lance or in upwelling molten material adjacent the plume. Thus, calcium residence time in the bath is unacceptably low.
In an attempt to overcome the above-mentioned problems, calcium has also been added to melts in steelmaking ladles in the form of a calcium metal-containing wire (clad or unclad) continuously fed through the upper surface of the melt. A major advantage of wire feeding is that large flows of gas are not needed, as in powder injection, to propel the calcium-containing material into the molten ferrous material. However, the high volatility of calcium hinders the attainment of an efficient utilization of the calcium added in surface wire feeding. If the wire does not penetrate to a sufficient depth below the surface before the calcium in the wire desolidifies, a low residence time and poor recovery of the calcium results along with a non-uniform treatment of the melt. It is particularly important that most or all of the input calcium remain unreacted until it descends below the depth at which the ferrostatic pressure is equal to the vapor pressure of calcium. This goal is difficult to achieve, even when a clad calcium metal-containing wire is employed. When calcium desolidifies at ferrostatic pressures lower than its vapor pressure, large calcium gas bubbles are formed that rise rapidly to the surface of the melt. The result is an inefficient, non-uniform treatment of the molten ferrous material and the generation of a large amount of turbulence at the surface of the melt.
U.S. Pat. No. 4,512,800, assigned to the applicant, discloses an apparatus and method for treating molten ferrous material with processing additives in wire form such as calcium containing wires directly into a quantity of molten material using a heat-resistant lance having an outlet disposable beneath the surface of the molten material. In such a lance apparatus, the wire is fed into a passage going through the lance and an inert gas is concurrently injected into the passage together with the wire to prevent clogging of the lance by solidification of molten material while agitating the molten material by gas bubble agitation.
The use of the lance allows the calcium wire to melt and react with the molten ferrous material at a depth below the surface of the molten bath at which the ferrostatic pressure is greater than the vapor pressure of calcium at the temperature of the molten ferrous material. Because of the buoyancy of the wire, resulting from its lower density than that of the melt, the calcium wire bends.
The desolidification of calcium at a ferrostatic pressure greater than its vapor pressure leads to the creation by melting of liquid calcium globules, which rise much more slowly through the melt (thus providing a much higher residence time) than do calcium gas bubbles. As these liquid globules slowly rise through the molten ferrous material, those that do not react with inclusions are eventually transformed into a very large number of small gas bubbles that do not generate excessive turbulence when they reach the surface of the melt. Furthermore, the lance is generally positioned such that these liquid calcium globules rise through a region of downwelling of the ferrous molten material to promote efficient utilization of the calcium additive.